1. Field of the Invention
The invention relates to a field device, for example, for a measurement device for determining a measured or process variable or an actuator for adjusting or controlling a process variable or a corresponding surrounding condition.
2. Description of Related Art
In modern process automation, processes are monitored or controlled with different field devices. Such field devices—designed as measurement devices or as actuators—are implemented either as stand-alone devices or they are connected, for example, via a field bus, with a process control system (PCS). Communication buses, for example, field buses according to the Profibus- or HART standard, are used for communication with such a PCS or a control room included by a PCS. Furthermore, field devices can still be differentiated by whether they have a common interface with two wires or different interfaces with a total of four (or more) wires for data communication and power supply. Accordingly, such devices are also referred to as two- or four-wire devices.
Despite all the efforts and improvements, it is or cannot always be out ruled that errors occur, that unforeseen events occur or that field devices are used outside their specifications. Furthermore, it may be advantageous or necessary that conditions are recorded, and thus, documented. Moreover, it is advantageous for improving the quality of the field devices, when detailed information about continuous operation is provided.
To gain access to the existing data in the field devices, for example, the European Patent EP 0 964 325 B1 discloses a method, in which status or diagnostic data from field devices is collected and transmitted to an evaluation unit. A problem here is that the amount of data provided by the field devices can possibly be very high, so that the data bus between the field devices and a control room can be very busy. This is particularly problematic when the field devices only have a limited amount of energy available. Moreover, such a method is generally not possible in field devices that are connected to a data bus. Moreover, it is not necessarily desired that all of the data, for example, concerning the process to be monitored and controlled, be transmitted via a field bus.
As an alternative, for example, the German Patent Application DE 10 2005 062 418 A1 describes a separate data logger that can be connected to a field device and allows local data logging. A disadvantage is that this is an additional component that has to be connected to the field device.